Modelling seed recruitment controls in an Alpine floodplain subject to hydropeaking

Hydropower production has different recognized impacts on river ecosystems. In particular, it alters the natural hydrological regime with extended low-residual flow conditions interrupted by rapid daily and sub-daily flow fluctuations, i.e. hydropeaking. Hydropeaking impacts both biotic and abiotic compartments: an increasing body of literature suggests that it can influence the physiological activity of plants, seed germination, and seedling growth, altering the chance of survival of several plant species.

Riparian vegetation is a key indicator of the status of river hydro-morphological processes. Several riparian plant species are nowadays endangered because of the degradation of river ecosystems worldwide, as a result of the exploitation of river resources. River floodplains, by hosting large amounts of biodiversity and habitat types, are crucial objectives for river management and restoration.

Vegetation establishment in floodplains and in-channel morphologies is linked to river hydro-morphodynamic processes: seeds of many riparian species are transported along the river by water, deposited on shorelines as the water level recedes, and establish depending on different environmental factors. The hydrological regime at seasonal-yearly scale (for example flood-drought seasonality), has recognized effects on seed recruitment.

In this study, we applied a vegetation recruitment model based on the Windows of Opportunity concept to study the main hydro-morphological controls on seed recruitment in an Alpine river subjected to hydropeaking. The study site is a small gravel-dominated floodplain of Moesa River (Switzerland). The model predicts potential colonization sites for vegetation after seed dispersal events by comparing water stress caused by water level fluctuations and time-varying plant resistance to inundations. We test alternative hydrological scenarios, comparing business-as-usual and no-hydropeaking conditions, and also different morphological configurations, using river topographical scans from different epochs (pre- and post- natural floods). We use a two-dimensional depth-averaged hydrodynamic model to simulate water levels in every scenario. The different hydro-morphological configurations are then fed into the seed recruitment model, to finally evaluate spatially distributed maps of successful rate of seed recruitment. Each hydrological and morphological scenario is tested also against different vegetation resistance to water stress, hence comparing stress-intolerant and stress-tolerant plant species. In addition, we qualitatively compared our results with an existing dataset of German Tamarisk (Myricaria germanica) dynamics in the floodplain.

Our results show the influence of vegetation resistance on the successful recruitment rate in terms of spatial extension and distribution. The influence of hydropeaking seems to be increased/smoothed depending on the hydrological year. Morphological variations due to natural floods appear to have relevant impact on vegetation dislocation, but less on total amount. Developing quantitative tools to simulate eco-morphodynamic river processes is supportive for both river managers and scientists. Eventually the understanding of key physical drivers of riparian vegetation dynamics in hydropeaking rivers is crucial for the conservation and restoration of functional river ecosystems.